JPH0119124B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an optical system which is a large aperture ratio photographic lens having a standard angle of view, and which has a shorter shooting distance than a normal lens. Conventionally, many large aperture ratio lenses have been corrected for aberrations based on infinity shooting, and it is known that the imaging performance deteriorates significantly when shooting at close range, and that the larger the aperture ratio lens, the more the various aberrations fluctuate. It is being In practical terms, the magnification when shooting at close range is about 1/10, and even with a large aperture ratio lens, the performance when shooting wide open is far from sufficient. Ta. In order to prevent this kind of deterioration in imaging performance, various so-called floating methods have been adopted, in which parts of the lens system are moved differently during close-up photography, in place of the conventional so-called whole-feeding method. Therefore, fluctuations in various aberrations in close-range shooting conditions can be reduced, and the shooting distance can be made shorter than in the case of the entire extension method. However, these known floating methods generally have complex intertwined structures, require lens movement that is difficult to achieve higher magnification, or have simple structures. However, image deterioration due to chromatic aberration was severe, and it was difficult to shorten the shooting distance, especially when using a large aperture ratio. An object of the present invention is to provide a large aperture ratio photographic lens with a standard angle of view as a lens for a single-lens reflex camera, but with less deterioration in chromatic aberration and various aberrations in close-up shooting conditions, and with a closer shooting distance than before. The goal is to provide a short version of the content. In the large aperture ratio photographic lens according to the present invention, the entire optical system is divided into two groups with an aperture in between, and both the front group and the rear group have at least one negative lens and have a composite positive refractive power. When moving from infinity to close-up photography, focusing is performed by extending the entire system toward the object side and widening the aperture space between the front and rear groups, and the composite focus of the entire system is When the focal length of the front group is ₁ and the focal length of the rear group is ₂ , _the distances satisfy the following conditions: 2.4< ₁ /<4.5 (1) 2.5< _1/2 <5.5 (2). When transitioning from infinity to close-up photography, let y be the amount of change in the aperture space between the front group and the rear group, and let the amount of change in back focus be x, and in the functional relationship y=f(x), It is optimal that the differential value dy/dx satisfies the following condition: 0.2<dy/dx<0.6 (3). The present invention achieves achromatization in each group by providing at least one negative lens in each of the positive front and rear groups, and also changes the aperture space between both groups during focusing, As a result, not only aberrations related to the reference wavelength in close-range photography but also chromatic aberrations, particularly off-axis chromatic aberrations, can be well balanced and corrected. In the conventional floating method, the partially movable lens does not have a degree of freedom for achromatization, making it impossible to correct on-axis and off-axis chromatic aberrations at the same time, and because it depends on changes in space other than aperture space. Although wavelength aberrations can be corrected, it is extremely difficult to properly correct off-axis chromatic aberrations, and this has been an obstacle to improving short-range performance with a simple configuration in the conventional floating method. It is. In addition, in the present invention, when shooting at close range, the distance between both groups, that is, the aperture space becomes large, so if the aperture is provided integrally with the rear group, the entrance pupil moves farther from the object and enters the lens system. This makes it easier to correct aberrations because the angle it makes with the optical axis of the light flux becomes smaller, and when the aperture is integrated with the front group, the exit pupil moves away from the image, making the angle of the light flux exiting the lens system smaller. Therefore, even in this case, aberration correction becomes easy. Therefore, based on the correction of chromatic aberrations as described above, by appropriately balancing the distribution of refractive power between the front and rear groups as shown below, it is possible to achieve good aberrations at close distances while maintaining a large aperture ratio. I was able to correct it. The present invention does not aim to achieve a magnification as high as that of a so-called microlens or macrolens that is capable of close-up photography at high magnification, but rather to achieve an even larger aperture ratio. However, it is an optical system that can take images at higher magnification than normal lenses. In order to suppress the occurrence of spherical aberration and coma aberration and to ensure a sufficient back focus length for single-lens reflex cameras, the ratio of the refractive power of the entire system to the refractive power of the front group is lower than that of a lens that can also perform high-magnification close-up photography. Since it is small, the front group can be made into a sufficiently bright optical system on its own that can withstand use. In addition, since the refractive power of the front group is weak, the light beam emitted from the object point on the optical axis passes through the front group and then becomes a divergent light beam and enters the rear group at a much lower photographic magnification than that of a so-called microlens. Band spherical aberration, coma aberration, and astigmatism begin to increase rapidly. When this light beam starts to become a divergent light beam, the burden of aberration correction begins to be placed on the rear group, so in order to maintain a large aperture ratio, it is necessary to keep the imaging magnification lower than that of a microlens. The refractive power distribution of the group is determined from this perspective. In other words, the limit state of the closest distance is a state in which the light flux that can become a divergent light flux is parallel to the optical axis in the aperture space or becomes a slightly divergent light flux. Regarding conditional expressions (1) and (2) according to the present invention, ₂ =
When ₁ = ∞, the refractive power of the front group is completely lost, which is not suitable for the present invention, but such an unrealized region is eliminated by the upper limit of equation (1). Equation (1) defines the distribution of refractive power necessary for a large aperture ratio for close objects.
If the lower limit of equation (1) is exceeded, it becomes difficult to secure sufficient back focus for a large-aperture single-lens reflex camera, and in terms of aberrations, the refractive power of the front group is too strong, resulting in significant annular spherical aberration. As a lens with a large aperture ratio, it is difficult to perform sufficiently good aberration correction even at short distances. On the other hand, if the upper limit of equation (1) is exceeded, it is easy to obtain a sufficient aperture space, but the overall length of the lens becomes too long, which is disadvantageous for compactness. In addition, at close range, the light flux from an on-axis object becomes a stronger divergent light flux after passing through the front group, so
The burden of aberration correction on the rear group increases, and in order to sufficiently correct aberrations such as annular spherical aberration, extroverted coma, astigmatism, and field curvature, a complex lens configuration must be adopted for the rear group. However, this is also disadvantageous for compactness. In addition, in general, in a lens system with a large aperture ratio, the aperture of each lens inevitably becomes large, so the edge thickness of each lens must also be sufficiently thick, and the center thickness of each lens also becomes large, which means that the principal point of the lens tends to enter the inside of the lens. In the present invention, the aperture space changes and becomes the smallest when shooting at infinity, so
We also attempted to secure aperture space using the conditions (1) and (2) above. The condition of equation (2) is the ratio of the refractive power of the rear group to the refractive power of the front group, and like equation (1), it is a condition that determines an appropriate distribution of refractive power, and it also ensures that the aperture space is wide and the back focus is sufficiently long. This is a supplementary conditional expression to equation (1) that is necessary to ensure. If the lower limit is exceeded, the refractive power of the first group becomes strong, making it impossible for the front group to withstand bright usage conditions, making it difficult to correct various aberrations, and shortening both the aperture space and the back focus, which is not desirable. If the upper limit is exceeded, the refractive power of the first group becomes too weak, similar to the upper limit of equation (1), so the light beam emitted from the optical axis object point passes through the front group and becomes a diverging light beam at a low imaging magnification and enters the rear group. This is not desirable because it makes it difficult to correct aberrations in the rear group. In the above optical system consisting of two positive front and rear groups, the average refractive index of the glass of the positive lens in the front group is N.
This value is preferably in the range of 1.75<N<1.81. The upper limit is because there is no glass with low dispersion and high refractive index, and the lower limit is because it becomes difficult to correct astigmatism, field curvature, and spherical aberration. Further, if the Abbe number of the glass of the negative lens in the front group is V, it is desirable that the range be 29.6<V<59. The lower limit is the flint system, which has a high refractive index, making it difficult to correct astigmatism and field curvature with this type of large aperture ratio lens. At the upper limit, the refractive index begins to decrease,
It becomes difficult to correct the annular spherical aberration, and the axial achromatization by the front group becomes insufficient. Then, as in condition (3) above, when going from an infinity shooting state to a close-up shooting state, if the amount of change in aperture space is y and the amount of change in back focus is x, then the functional relationship y = (x) is obtained. Hold and focus.
At this time, it is desirable that the first-order differential value be in the range of 0.2<dy/dx<0.6. The coordinate vertices of the function are always photographed at infinity. If the lower limit is exceeded, the imaging performance will be close to that in close-range photography using the entire extension method, and the effects of the present invention cannot be achieved. If the upper limit is exceeded, spherical aberration, coma aberration, and astigmatism will be overcorrected, contrary to when they do not float, making it difficult to achieve a desired aberration balance. In the configuration of the present invention as described above, specifically, a so-called modified Gauss type lens system is adopted. That is, as shown in Figs. 1, 3, and 5, the front group _G1 is arranged in order from the object side: the first positive lens L1, the positive meniscus lens _L2 with its convex surface facing the object side, and the first positive lens _L2 , which is also on the object side. The rear group G2 consists of a meniscus lens _L3 with a convex surface facing the object side, and the rear group _G2 consists of a meniscus lens _L4 made up of a negative lens and a positive lens with a concave surface facing the object side, and a second positive lens _L5 . The rear group G ₂ is further provided with a third positive lens L ₆ to make it brighter. In such a specific configuration, the shape factor of the first positive lens L ₁ closest to the object in the first group is δ=(γ ₂
+γ ₁ )/(γ ₂ −γ ₁ ) (where the radius of curvature of the front surface of this lens L ₁ is γ ₁ and the radius of curvature of the rear surface is γ ₂ ), it is desirable that the range is 0.9<δ<1.3. If the lower limit is exceeded, the astigmatism becomes too negative and the coma aberration becomes large, and if the upper limit is exceeded, the distortion becomes too negative and it becomes impossible to maintain balance significantly more at close range than when shooting at infinity, which is not desirable. Further, if the refractive index of the positive meniscus lens L ₂ in the front group is N ₂ and the refractive index of the negative meniscus lens L ₃ is N ₃ , it is desirable to have a relationship of N ₂ >N ₃ . Large aperture ratio lenses use glass with a high refractive index, and the refractive power of the negative lens component becomes weak, so the Petzval sum becomes excessively positive and astigmatism tends to occur. It is desirable to reduce the Petzval sum by using glass with a low index, and to strengthen this tendency within a range where zonal spherical aberration does not occur. Furthermore, in this case, since it is a negative lens with a low refractive index, one with low dispersion can be used, which allows off-axis chromatic aberration to be positively displaced at close range, and between infinity and close range. This is desirable because it allows for smaller correction of fluctuations in off-axis chromatic aberration during distance photography. Examples according to the present invention will be described below. All of the examples are for 35mm cameras, and the aperture is configured to always move together with the rear group. Focal length = 51.6, total angle of view 2ω = 45.43°. The first embodiment has an F number of 1.8, and as shown in Fig. 1, Fig. 1a shows the lens arrangement when photographing at infinity, and Fig. 1b shows the lens arrangement when photographing a close-up object. be. The specifications of the first embodiment are shown in Table 1, and the aberration diagram is shown in FIG. Figure 2a shows the aberration when photographing an object at infinity, and Figure 2b shows the object distance d ₀
=286.9196 and photographing magnification β=-0.2. This is an example in which the average rate of change regarding floating is Δy/Δx=0.4764, and the refractive power of the front group is relatively strong. In addition, despite floating, changes in off-axis chromatic aberration were suppressed to the same level as with the conventional full-floating system. In the second embodiment, as shown in FIG. 3, a positive meniscus lens _L6 is added to the rear group to further increase the aperture ratio. A modified Gauss type lens is also used, and FIG. 3a shows the lens arrangement when photographing at infinity, and FIG. 3b shows the lens arrangement when photographing an object at close range. Since the F number is set to 1.4, the close range is longer than in the first embodiment. The specifications of the second example are shown in Table 2, and the aberration diagram is shown in FIG. FIG. 4a is an aberration diagram when photographing an object at infinity, and FIG. 4b is an aberration diagram when the object distance d ₀ =364.224 and the photographing magnification β=−0.15. The average rate of change regarding floating is Δy/Δx=0.3915. Furthermore, in this example, distortion is well corrected despite the large aperture ratio lens. This is due to the refractive power and shape of the positive lens group in the front group, which causes distortion in the positive direction, which reduces the overall system. The third embodiment employs a modified Gaussian type similar to the second embodiment, as shown in FIG. FIG. 5a is a lens arrangement diagram when photographing at infinity, and FIG. 5b is a diagram showing the lens arrangement when photographing an object at close range. Like the second embodiment, the F number is 1.4.
The specifications of the third embodiment are shown in Table 3, and the aberration diagram is shown in FIG. FIG. 6a is an aberration diagram when photographing an object at infinity, and FIG. 6b is an aberration diagram when object distance d ₀ =354.649 and photographing magnification β=−0.15. The average rate of change with respect to floating is Δy/Δx=0.2306, and this is an example of an arrangement in which the refractive power of the front group is biased toward a relatively weak direction.

【table】

【table】

【table】

【table】

[Table] According to the present invention configured as described above, although it is a large aperture ratio photographic lens having a standard angle of view as a lens for a single-lens reflex camera, the deterioration of chromatic aberration and various aberrations is small in close-range shooting conditions. It is possible to obtain a large aperture ratio photographic lens with a shorter shooting distance than before.

[Brief explanation of drawings]

FIG. 1 is a lens arrangement diagram of the first embodiment of the present invention, FIG. 1a is a lens arrangement diagram when photographing an object at infinity, FIG. Fig. 2 is an aberration diagram of the first embodiment;
Figure a is an aberration diagram when photographing an object at infinity, Figure 2 b is an aberration diagram when object distance d ₀ = 286.9196 and photographing magnification β = -0.2, and Figure 3 is a lens arrangement diagram of the second embodiment. Figure 3a is a lens layout diagram when photographing at infinity, Figure 3b is a lens layout diagram when photographing a close-up object, Figure 4 is an aberration diagram of the second embodiment, and Figure 4a is a diagram of the lens layout when photographing an object at close range. Aberration diagram when photographing an object at infinity, Figure 4b is the object distance
An aberration diagram when d ₀ = 364.224 and photographing magnification β = -0.15. Figure 5 is a lens arrangement diagram of the third embodiment.
Figure a is a lens arrangement diagram when shooting at infinity;
Figure b is a lens arrangement diagram when photographing a close-up object, Figure 6 is an aberration diagram of the third embodiment, Figure 6a is an aberration diagram when photographing an object at infinity, and Figure 6b is an object distance diagram.
It is an aberration diagram when d ₀ =354.649 and imaging magnification β = −0.15. [Explanation of symbols of main parts] G ₁ ...front group, G ₂ ...
rear group.

Claims (1)

[Scope of Claims] 1. A front group having a positive refractive power, a rear group also having a positive refractive power, and a diaphragm provided between the two groups, and the front group is located on the object side. and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a convex surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side, and a meniscus lens with a concave surface facing the object side and a positive lens, and when transitioning from a state of photographing an object at infinity to a state of photographing a close object, both groups are moved toward the object side while increasing the distance between the two groups where the aperture is arranged. When the composite focal length of the entire system is f, the focal length of the front group is _f1 , and the focal length of the rear group is _f2 in the infinity shooting state, 2.4 < f. ₁ / f < 4.5 (1) 2.5 < f ₁ / f ₂ < 5.5 (2) In addition to satisfying the following conditions, the aperture between the two groups is When the amount of change in space is y and the amount of change in back focus is x, in the functional relationship y=f(x), the first differential value dy/dx satisfies the condition 0.2 < dy/dx < 0.6 (3) A close-range corrected large aperture ratio photographic lens.